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<title>Why attributes set with stream manipulators do not participate in filtering?</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="log.rationale.why_attribute_manips_dont_affect_filters"></a><a class="link" href="why_attribute_manips_dont_affect_filters.html" title="Why attributes set with stream manipulators do not participate in filtering?">Why
      attributes set with stream manipulators do not participate in filtering?</a>
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<p>
        One can add attributes to log records in the following way:
      </p>
<pre class="programlisting"><span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">logger</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">add_value</span><span class="special">(</span><span class="string">"MyInt"</span><span class="special">,</span> <span class="number">10</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">logging</span><span class="special">::</span><span class="identifier">add_value</span><span class="special">(</span><span class="string">"MyString"</span><span class="special">,</span> <span class="string">"string attribute value"</span><span class="special">)</span>
    <span class="special">&lt;&lt;</span> <span class="string">"Some log message"</span><span class="special">;</span>
</pre>
<p>
        However, filters will not be able to use MyInt and MyString attributes. The
        reason for this behavior is quite simple. The streaming expression is executed
        <span class="emphasis"><em>after</em></span> the filtering takes place and only <span class="emphasis"><em>if</em></span>
        the filter passed the log record. At this point these attributes have not
        been added to the record yet. The easiest way to pass attributes to the filter
        is to use scoped attributes or tags (see <a class="link" href="../detailed/attributes.html#log.detailed.attributes.related_components.scoped_attributes" title="Scoped attributes">here</a>).
      </p>
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<td align="right"><div class="copyright-footer">Copyright © 2007-2021 Andrey Semashev<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>).
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